Molecular orbital approach to the electronic structure of CdS ultrafine particles

The electronic structure of CdS ultrafine particles was studied by the semiempirical molecular orbital (MO) method. A practical procedure to perform MO calculations on a very large molecule which contains roughly one thousand atoms was established. The aimed electronic structure was obtained and characterized. A tendency which corresponds to the so-called size quantization effect usually observed in semiconductor ultrafine particles was confirmed. An electronic transition energy, which was almost comparable to experimental data, was obtained. The influence of surface structures on the electronic state was directly calculated by the MO method. Low-coordinated Cd atoms were found to produce surface states. A surface defect site associated with excess Cd was simulated by introducing a sulfur vacancy. A new MO that originated from the defect site, which may act as a trapping site of excited electrons, was identified.